Ultradeep drillship will react to heave with electric- compensating drawworks

Dean E. Gaddy
Drilling Editor

Conoco Inc. and Reading & Bates Corp. are developing a new ultra-deepwater drillship which will utilize an innovative electric, active-heave compensating drawworks to counterbalance vessel heave (Fig. 1 [19,408 bytes]).

Vertical motion will be counteracted by spooling wire either off or back onto the drawworks drum. For instance, if the ship heaves upward 3 ft, then 42 ft of wire will be reeled off the drum using a 14 line block-and-tackle configuration.

This system will have two advantages over conventional traveling-block and crown-mounted compensators:

First, it will electronically sense the relative movement of the vessel, eliminating the mechanical need for changes in weight or pressure to compensate for vessel movement. Second, the full-rated load of the traveling equipment can be utilized during compensation because the system will be an integral part of the drawworks. This feature will be important for operations in ultra-deep water when running loads approaching the weight limits of the traveling equipment.

Sensors will be located on the hull at the moonpool, in the derrick, and on the dead-line. The hull's sensor will provide input to calculate the position of the rig floor as it moves. The derrick's sensors will check the position of the blocks, and the dead-line's sensor will monitor the hook-weight.

Input from the sensors will be processed by a computer. The command logic will then be outputted to programmable logic controllers which will then activate the ac motors, thereby providing power to the drawworks.

The drawworks will be powered by six General Electric ac motors (GEB 20AC), each generating 1,150 hp for a total of 6,900 continuous hp (Fig. 2 [11,220 bytes]). This power will be used for hoisting, thereby eliminating the need for an active resistance brake normally needed to lower heavy loads.

Brett Borland, drilling engineer for Conoco said that there is a risk in using the electric, active-heave compensation system because of the tremendous mass involved while landing the BOP stack in 7,500 ft of water. The total system mass during this operation including riser, buoyancy, and BOP stack is over 5 million lb.

He said the Gulf of Mexico (GOM) will be a good training ground for the system because normal weather systems are less severe than in the North Sea. Typical heave in a 1-year GOM storm for the Conoco/Reading & Bates drillship will be less than 4 ft.

Two companies are involved with the design and construction of the system. Dreco Inc. will manufacture the 6,600 hp direct-drive ac drawworks, while Hitec ASA is responsible for the control system, program logic, and final system design.

The drillship will be capable of drilling in 10,000 ft of water to a depth of 25,000 ft, and is slated to evaluate 60 federal blocks in the GOM in 2,000-9,000 ft of water (OGJ, Nov. 11, 1996, p. 34). The drillship construction is scheduled to be completed in September 1998.